Two years after young adults (early twenties) quit smoking marijuana, researchers found changes in the sub-cortical regions of their brains associated with memory and reasoning, indicating the long-term effects of chronic use. They were also found to perform poorly on memory tests.

"The study links the chronic use of marijuana to these concerning brain abnormalities that appear to last for at least a few years after people stop using it," Matthew Smith, an assistant research professor in psychiatry and behavioral sciences at Northwestern University Feinberg School of Medicine and lead study author, said in a press release. "With the movement to decriminalize marijuana, we need more research to understand its effect on the brain."

According to Substance Abuse and Mental Health Services Administration, the marijuana usage by teens, aged 12 to 17, was reduced to 9.5 percent last year from almost 12 percent in 2002. These figures are predicted to increase due to the legalization of marijuana, allowing more access to underage smokers.

For the study, Smith's team compared MRI scans of a controlled group (44) with 10 people with cannabis use disorder, 15 with cannabis use disorder and a diagnosis of schizophrenia, and 28 with schizophrenia but no reported use of marijuana in the past.

The participants, who were aged mid-20s at the time of the testing, had been heavy users as teens.

Through the MRI scans, the researchers found that heavy users displayed abnormalities in all the three brain regions (striatum, thalamus and globus pallidus), irrespective of whether they had schizophrenic disorder or not. The volume of the thalamus was found greatly reduced in heavy users.

Then the participants were asked to undertake four memory tests, like recollecting a sequence of numbers. Heavy users fared badly than healthy controls and non-using schizophrenics.

"The abuse of popular street drugs, such as marijuana, may have dangerous implications for young people who are developing or have developed mental disorders," said co-senior study author John Csernansky, M.D., chair of psychiatry and behavioral sciences at Northwestern University Feinberg School of Medicine and Northwestern Memorial Hospital. "This paper is among the first to reveal that the use of marijuana may contribute to the changes in brain structure that have been associated with having schizophrenia."